The present invention relates to a circuit arrangement for a brake system with electronic control of the brake force distribution to the front axle and the rear axle, hereinbelow referred to as EBV, including electrically controllable hydraulic valves inserted in the pressure fluid conduits leading to the rear-wheel brakes and in return conduits, wheel sensors to determine the wheel rotational behavior, and an electronic circuit which evaluates the sensor signals, generates signals to actuate the hydraulic valves and includes circuits which determine the deceleration of the rear wheels and the deceleration of the vehicle or a reference quantity representative of the deceleration of the vehicle in approximation, and which compare these deceleration values.
German patent application No. 33 06 611 (GB-A-2 135 413) discloses a circuit arrangement of this type. This application relates to a method of controlling the brake force distribution which is based on measuring and comparing the brake slip on the front wheels and the rear wheels. The brake slip on the rear wheels is limited to the value of the brake slip on the front wheels or to a somewhat smaller value. The braking pressure on the rear axle is reduced in response to a signal which is indicative of an imminent locked condition of the rear wheels and produced by comparison of the rear-wheel deceleration with the vehicle deceleration.
German patent No. 33 23 402 discloses an anti-lock brake system including electromagnetically operable hydraulic valves which are also used to control the brake force distribution. For this purpose, an inlet valve is inserted into each pressure fluid conduit to the rear-wheel brakes. Actuation of the inlet valve permits controlling the pressure increase in the rear-wheel brakes also in a so-called partial braking operation, i.e., prior to the commencement of anti-lock control. To control or adjust the brake force distribution, an outlet valve can be used to reduce the braking pressure in the rear-wheel brakes. It is preferred to limit the brake slip on the rear wheels to a defined percentage, preferably 85-97% of the brake slip of the front wheels, by actuation of the rear-wheel valves.
A brake system with electronic brake force distribution (EBV) of this type obviates the need for mounting a conventional mechanical or hydraulic braking pressure reducing valve or braking pressure control valve which has previously been used to prevent overbraking of the rear wheels due to a static or dynamic axle load shift. This reduces the expenditure in manufacturing the brake system. In addition, the components which are required in an electronic anti-lock system, in particular the wheel sensors and the hydraulic valves, may also be used for the EBV function.
A shortcoming of these known brake systems with EBV function is that the wheel speed must be measured with great accuracy to determine the actual slip. A defined amount of slip already is involved due to the inevitable differences in the rolling circumferences of the individual wheels. Also, differences in wheel speeds occur during cornering, which the control erroneously interprets as slip. Upon failure of one of the front-wheel brakes, special provisions are required when an EBV function on the basis of the difference between the slip of the front wheel and the rear wheel is performed to prevent the braking pressure in the rear-wheel brakes from being restricted to an insufficient amount of braking pressure.